Mechanical factors and the electrocardiogram

Diastolic (dys-)function and electrophysiology

Cross-talk between cardiac electrical and mechanical function is a bidirectional process: The origin and spread of electric excitation govern cardiac contraction and relaxation, while the mechanic environment provides feedback information to the heart's electric behavior. The latter tends to be unduly disregarded by the medical community. This article reviews experimental findings on the effects of diastolic mechanics on cardiac electrophysiology, and describes physiological correlates, clinical manifestations, and therapeutic utility of cardiac mechanic stimulation in humans.

Stretch-induced changes in heart rate and rhythm: clinical observations, experiments and mathematical models

Clinical and research data indicate that active and passive changes in the mechanical environment of the heart are capable of influencing both the initiation and the spread of cardiac excitation via pathways that are intrinsic to the heart. This direction of the cross-talk between cardiac electrical and mechanical activity is referred to as mechano-electric feedback (MEF). MEF is thought to be involved in the adjustment of heart rate to changes in mechanical load and would help to explain the precise beat-to-beat regulation of cardiac performance as it occurs even in the recently transplanted (and, thus, denervated) heart. Furthermore, there is clinical evidence that MEF may be involved in mechanical initiation of arrhythmias and fibrillation, as well as in the re-setting of disturbed heart rhythm by 'mechanical' first aid procedures. This review will outline the clinical relevance of cardiac MEF, describe cellular correlates to the responses observed in situ, and discuss the role that quantitative mathematical models may play in identifying the involvement of cardiac MEF in the regulation of heart rate and rhythm.

Changes in morphology of the paced QRS complex related to atrial contraction

A patient with 2:1 AV block underwent temporary ventricular pacing. All the paced stimuli resulted in ventricular capture, but a marked variability in morphology of the paced QRS complexes occurred. Two different types of paced QRS complex (labeled A and B) were recognized. Type B complexes were manifest only when the pacing stimulus was preceded by a sinus P wave within a time interval ranging from 0.15 to 0.52 sec. The P wave-induced changes in morphology of the paced QRS complexes were interpreted as due to displacement of the pacing ventricular lead caused by atrial systole.

Mechanically mediated pacemaker pseudomalfunction

A case is described of pacemaker pseudomalfunction caused by intermittent displacement of the ventricular lead. The displacement was shown to be caused by critically timed atrial contractions, dislodging the looped ventricular lead, and resulting in intermittent failure to stimulate the heart.

T wave changes in the right precordial leads in seven- and eight-year-old children during a Valsalva manoeuvre

The T waves in the right precordial leads were analyzed in 106 eight-year-old and 90 seven-year-old unselected healthy children. Recordings during relaxation were compared with those obtained during a Valsalva manoeuvre. In the relaxed state only three children in each age group showed a positive T wave in lead V1, while during the Valsalva manoeuvre 90 of the 106 eight-year-old and 55 of the 90 seven-year-old children converted to a positive T V1. The difference is highly significant in both age groups and both sexes (p much less than 0.001). Minor changes towards more positive T V2 and T V4 were also found.

Atrial systole and ineffective pacemaker stimuli

An unusual case of pacemaker failure induced by atrial activity is presented. The pacemaker stimulus was ineffective for ventricular capture not only with low energy output (decreased pulse duration) of the generator, but also with pulse width two to three times the stimulation threshold. The two possible mechanisms of this phenomenon, one electrotonic and the other mechanical, are discussed. This report strongly supports the mechanical hypothesis.

Early failures of permanently implanted transvenous pacemaker systems: spontaneous recovery of sensing function in three of twelve cases

An analysis of 12 early pacemaker failures in a series of 128 transvenous implants is reported. Spontaneous recovery, maintained during long-term follow-up of 3 patients with isolated transient sensing failure, suggested the possibility of a transient loss of sensing voltage amplitude, or slew rate (dv/dt), or both. Conservative management is frequently indicated in these cases. In 62 patients in whom a "tined" lead was used, there were no early pacemaker failures.

R on T or R on P phenomenon? Relation to the genesis of ventricular tachycardia

Continuous electrocardiographic monitoring of 225 patients with acute myocardial infarction was performed during the initial 48 hours after admission. Two hundred twelve episodes of ventricular tachycardia occurred in 49 subjects, and 8 patients had primary ventricular fibrillation. Most cases of ventricular tachycardia were associated with late coupling of premature ventricular complexes. Of the 212 instances of ventricular tachycardia, 42 (20 percent) were initiated by a premature complex on the T wave (R on T) (R-R'/Q-T less than 1), and 93 (44 percent) had initiating premature complexes that occurred directly after onset of the sinus P wave (R on P). Of eight episodes of ventricular fibrillation, seven were initiated by a premature ventricular complex and in four of these there was associated R on T phenomenon. The influence of atrial contraction and myocardial stretch on reentry or ectopy is proposed as a possible explanation for the relatively high incidence rate of ventricular tachycardia observed after the onset of the sinus P wave.

Dynamic electrocardiography. II. A crucial test of the electromechanical QRS wave theory

The dynamic electromechanical electrocardiogram hypothesis, that QRS voltage fluctuations can be used as a simple noninvasive transducer of cardiac mechanical function, has been subjected to a crucial experiment. Under direct vision, transient modifications of the end-diastolic volume of the baboon heart were produced and photographed. Sequential obstructions to filling (by vena caval compression) and to emptying (by aortic compression), and vice versa, significantly distorted the size and shape of the heart. The instantaneous effects of these manipulations on the amplitudes of the R and S waves were evaluated in electrocardiograms recorded from electrodes glued to selected pericardial and epicardial sites. Major QRS voltage deviations occurred in the perircardial leads. Manipulations increasing the left ventricular volume increased the S wave and reduced the R wave, while those decreasing heartsize had the opposite effect. These findings refute the null hypothesis, that the electrocardiogram is not an indicator of mechanical function. No changes of the QRS waves in the epicardial lead were detected, supporting the concept that displacement relative to the recording electrode is the basis of the QRS CHANGEs due to heartsize variation. The results negate the classical concept that the electrocardiogram does not reflect cardiac mechanical function, and strongly corroborate the dynamic electromechanical electrocardiogram hypothesis.

Mechanical stimulation of the heart: its therapeutic value in tachyarrhythmias

This report presents the experience obtained with the use of mechanical stimulation of the heart by external ("chest thump") and internal (stimulation via catheter) means in the treatment of tachyarrhythmias. The chest thump was used primarily for termination of ventricular tachyarrhythmias. Endocardial stimulation of the right and left ventricles and right atrium via catheters was used for both ventricular and supraventricular arrhythmias. The experience reported includes 68 patients, 19 of whom were treated successfully with mechanical stimulation and 49 of whom required other types of treatment. Of the 32 patients with 33 episodes of atrial tachycardia, eight episodes (24%) were treated successfully (three episodes by right atiral stimulation and five by right ventricular stimulation). Of the four patients with five episodes of junctional tachycardia, three episodes (60%) were treated successfully (one episode by right atrial stimulation and two by right ventricular stimulation). Of the 32 patients with ventricular tachycardia who experienced 37 episodes, 15 episodes (41%) were treated successfully (three by right ventricular stimulation, two by left ventricular stimulation, and ten by chest thump). In two of these patients, the thump was applied to an area of paradoxical pulsation.